1. The Mishnah (Soṭah vii. 6; Tamid vii. 2) says: "In the Sanctuary the name of God [in the three blessings, Num. vi. 24-26] is to be pronounced in the Priestly Benediction as it is written []; but outside the Sanctuary it must be given the euphemistic pronunciation []." 

2. Ṭarfon's account, that the voice of the high priest was drowned by the song of the other priests, also confirms the synchronous statement (Yer. Yoma 40b) that in former times the high priest uttered the Name with a loud voice, but that subsequently, when immorality had become more and more prevalent, he lowered his voice lest the Name should be heard by those unworthy to hear it.

3. The Mishnah (Berakot, end) mentions also an utterance of the Tetragrammaton outside the Sanctuary which was permitted and even commanded, saying that "it was ordained that the name of God should be used in the ordinary forms of greeting, which were the same as those exchanged between Boaz and the reapers [Ruth ii. 2], or the salutation of the angel to Gideon [Judges vi. 12]."

 4. A haggadist of the third century, Abba bar Kahana, states (Midr. Teh. on Ps. xxxvi., end) that "two generations used the Shem ha-Meforash, the men of the Great Synagogue and those of the period of the 'shemad' [the Hadrianic persecution]." According to Sanh. vii. 5, actual blasphemy is committed only when the blasphemer really pronounces the Tetragrammaton ("Shem ha-Meyuḥad"; comp. Sifra, Emor, xix. [ed. Weiss, p. 104d]).These details indicate that the long-sanctioned dread of uttering the Shem ha-Meforash was by no means without exceptions, and that the correct pronunciation was not unknown. Abba Saul (2d cent.) condemned the profanation of the Tetragrammaton by classing those "that speak the Name according to its letters" () with those who have no part in the future world (Sanh. x. 1); and according to 'Ab. Zarah 17b, one of the martyrs of Hadrian's time, Hananiah b. Teradion, was burned at the stake because he so uttered the Name. A Palestinian amora of the third century (Mana the Elder) mentions "as, for instance, the Samaritans who swear"; he meant thereby that in their oaths the Samaritans pronounce the Tetragrammaton exactly as it is written. According to Theodoret, the Greek Church father, who flourished in the fifth century, they gave it the sound of Ἰαβέ (see Löw, "Gesammelte Schriften," i. 193). (Jewish Encyclopedia)

Naḥman b. Isaac, a Babylonian amora of the fourth century, said (Pes. 50a): "The future world is not like this world. Here the name of God is written, and read ; there it is also read ."

From the earliest times YHVH has been an extremely important element in Jewish mysticism. According to the Sefer Ḥanoch, it was Hillel who transmitted the name of God to the generations after Ezra, while Abbahu and Ze'era (3d and 4th cents.) and the "men of faith" ("anshe emunah") are mentioned as possessing this knowledge after Hillel. There are several other names, in addition to the Tetragrammaton, which are designated according to the number of their letters, as the Twelve-Lettered and the Forty-two Lettered Name, and the Seventy-two Lettered Name. The 72 letter name is a permutation of Exodus 14:19-21 (see below). The 42 letter name is constructed of the first letter of every word of the prayer "Ana B'Koach" featured in many Orthodox siddurs. A popular eight lettered name is derived from combining YHVH with ADNAI, so one gets YAHDVNHAI (yud-alef-daled-vav-nun-heh-yud). There is also the popular contractions of the divine name, like YH, YY, or H'. There is also the 304, 805 letter name of God- the Chumash. 

When Rebbe Nachman of Breslov was a young boy, sitting in front of his own teacher, he wanted to fulfill the verse "I have set YHVH always before me". He made great efforts to picture the four letter name of the Holy One, blessed be, before his eyes. Because his thoughts were occupied with this he did not know what he was learning, and his teacher became angry. Nevertheless, all the days of his youth (despite focusing on the divine name) he was able to engage in very childish behavior, playing games, jumping, and wandering around." (Shivchei Moharan 2)

Our sages say that the light of the Creation refers to the deeds of the righteous (Midrash Bereishit Rabbah 2:5). The explanation of this is that a wise person is able to eat good food and enjoy things and still be an ascetic. Similarly he is able to look at any place and see just the place where he is. That is: he should always see the shem hameforash (YHVH), which is inside each thing. This is the secret meaning of the verse, "I have set YHVH always before me", and of the Rabbinic teaching, "Do not look at the pot but at what is inside it." For the essence of everything in the universe is the spiritual within it, which comes from the Holy One, blessed be, who gives it meaning and keeps it in existence. (Likkutei Yekarim, R' Pinhas of Koretz [1726-1791])   

Rabbi Schneur Zalman of Liadi said: "Bereishit bara elohim et"....This should be read "In the beginning Hashem made "et", aleph-taf, the Alefbet. With the Alefbet he made everything else." 

Rabbi Shimon Bar Yochai said all the letters of the alefbet are either male or female and they come together to form a unity. This is the secret of the upper waters and the lower waters, which are really one, and form a perfect union. Because of this someone who knows them and is careful of them has a meritorious portion in the world to come. They are the underlying principle of the perfect unity...the letters are spread out on all sides to bind everything into a unity. For there are letters in which the secret of femininity resides and letters in which the secret of masculinity resides, and they ind everything together and they become one. This is the secret of the complete divine name. (Zohar 1:159a, Vayetse).    

Know that there is no body, nefesh, ruach, or neshama, that has not been created through the 22 letters of the Torah. Each of them possesses the the 22 letters of the Torah...the zaddikim, however, are able through the power of their souls to make them shine out from within the skin of people's faces. (Chesed Le-Avraham, 4:6)